The present invention relates to a plasma doping method for doping impurities into the surface of a solid sample such as a semiconductor substrate, and to an apparatus for implementing this method.
A technique for doping impurities into the surface of a solid sample is disclosed, for example, in the prior art of the U.S. Pat. No. 4,912,065, wherein a plasma doping method is implemented such that impurities are ionized and then doped into a solid material at a low energy.
A plasma doping method of the prior art method for doping impurities is described below with reference to FIG. 14.
FIG. 14 shows a schematic configuration of a plasma doping apparatus used in the prior art plasma doping method. In FIG. 14, a vacuum chamber 100 accommodates a sample electrode 106 for placing a sample (work) 109 such as a silicon substrate thereon. A gas supplying apparatus 102 for supplying doping material gas such as B2H6 containing a desired element and a pump 103 for evacuating the vacuum chamber 100 are provided outside the vacuum chamber 100. These apparatuses allow the vacuum chamber 100 to be maintained at a predetermined pressure. A microwave is emitted from a microwave waveguide 119 into the vacuum chamber 100 through a quartz plate 107 as a dielectric window. By a mutual action of this microwave and a DC magnetic field generated by an electric magnet 114, electron cyclotron resonance plasma is produced within a region encompassed by a dash-dotted line 120 in the vacuum chamber 100. The sample electrode 106 is connected to a high frequency power supply 110 through a capacitor 121, so that the electric potential of the sample electrode 106 is controlled.
In the plasma doping apparatus having the above-mentioned configuration, the doping material gas such as B2H6 introduced in the vacuum chamber 100 from the gas supplying apparatus 102 is made into the plasma state by plasma generating means comprising the microwave waveguide 119 and the electric magnet 114, so that boron ions in the plasma 120 are doped into the surface of the sample 109 by means of the electric potential provided from the high frequency power supply 110.
On the sample 109 doped with the impurities as mentioned above, a metallic wiring layer is formed in another process. Then, a thin oxide film is formed on the metallic wiring layer in a predetermined oxidizing atmosphere. After that, gate electrodes are formed on the sample 109 by a CVD apparatus and the like, so that MOS transistors, for example, are obtained.
The doping material gas such as B2H6 containing impurities express electrical activity when doped into a sample such as a silicon substrate, however there is a problem that such a gas is generally hazardous for human body.
Further, in the plasma doping method, all of the substances contained in the doping material gas are doped into the sample. Description is made in the case of the doping material gas composed of B2H6, for example. Only the boron works as effective impurities in the doped state, however hydrogen is also doped into the sample simultaneously. This doping of hydrogen into the sample causes the problem that lattice defects are generated during the subsequent heat treatment such as an epitaxial growth process.
For the purpose of resolving these problems, in another prior art method disclosed in JP-A Hei 09-115851, an impurity solid material containing the substance of impurities that express electrical activity when doped into a sample is placed in a vacuum chamber, while inert gas plasma is generated in the vacuum chamber. The impurities are emitted from the impurity solid material and are sputtered by the ions of the inert gas plasma. FIG. 15 shows the configuration of a plasma doping apparatus used in this prior art plasma doping method. In FIG. 15, a vacuum chamber 100 accommodates a sample electrode 106 for placing a sample 109 composed of a silicon substrate thereon. A gas supplying apparatus 102 for supplying the inert gas and a pump 103 for evacuating the vacuum chamber 100 are disposed outside the vacuum chamber 100. These apparatuses allow the vacuum chamber 100 to be maintained at a predetermined pressure. A microwave is emitted from a microwave waveguide 119 into the vacuum chamber 100 through a quartz plate 107 as a dielectric window. Due to a mutual action of the microwave and a DC magnetic field generated by an electric magnet 114, the electron cyclotron resonance plasma is produced within a region encompassed by a dash-dotted line 120 in the vacuum chamber 100. The sample electrode 106 is connected through a capacitor 121 to a high frequency power supply 110, so that the electric potential of the sample electrode 106 is controlled. An impurity solid material 122 containing impurity element such as boron is placed on a solid material holding bed 123. The electric potential of the solid material holding bed 123 is controlled by a high frequency power supply 125 connected thereto via a capacitor 124.
In the plasma doping apparatus having the above-mentioned configuration, the inert gas such as argon (Ar) introduced from the gas supplying apparatus 102 is ionized into the plasma state by plasma generating means comprising the microwave waveguide 119 and the electric magnet 114. A part of impurity atoms sputtered from the impurity solid material 122 into the plasma 120 are ionized and then doped into the surface of the sample 109.
Nevertheless, both of the above-mentioned prior art methods shown in FIG. 14 and FIG. 15 still have the problem that low density doping is not achieved stably and that the reproducibility of the processing is poor. When low density doping is performed using the doping material gas, it is required that the pressure of the vacuum chamber is reduced, and the partial pressure of the doping material gas is made low. For the purpose of the latter, in general, the doping material gas is diluted with helium, which is an inert gas. This is because helium ions have a lower sputtering yield and hence have the advantage that ion irradiation damage to the sample caused by the ions is suppressed. Nevertheless, helium also has the disadvantage that the start of its discharge is difficult at lower pressures. There is a difficulty in processing in desired low doping conditions.
Similarly, even when low density doping is performed using the impurity solid material in place of the doping material gas, the pressure of the vacuum chamber is reduced. In the case that argon is used as the inert gas, although the start of discharge at lower pressures is easier than the case of helium, processing in desired low density doping conditions is still difficult. This difficulty is essentially the same as the case of the use of the doping material gas.
On the other hand, JP-A 2000-309868 discloses a method in a sputtering apparatus using argon gas, wherein the pressure of the vacuum chamber is increased in a plasma generating step, so as to ensure the generation of the plasma. Nevertheless, this method is required to raise the pressure, and is not directly applicable to the plasma doping process, which is extremely sensitive to impurities.
An effective method for improving the generation-quality is to change discharge conditions so as to increase the pressure of the vacuum chamber in the generation step, and to decrease the pressure in the doping step. Nevertheless, the change in the pressure causes a substantial change in the discharge impedance. This impedance change cannot be sufficiently rapidly tracked by a matching circuit used for impedance matching of the high frequency electric power. This causes a problem of the generation of a large reflected power. More specifically, a typical matching circuit comprises, as variable impedance elements, two variable capacitors (or stubs in case of a microwave) having a mechanical section which is driven by a motor. Thus, impedance matching adjustment typically takes one second or longer because of the mechanical rotation by the motor. The reflected power degrades the reproducibility of the processing. The reflected power is liable to generate a noise and hence erroneous operation of apparatuses occurs. In a worse case, the rotation (movement) of the variable capacitors (stubs) overruns the appropriate position, and causes the extinction of the plasma.
Further, once a microwave or a high frequency electric power is supplied to the plasma doping apparatus, the matching circuit provided between the high frequency power supply and the plasma generating apparatus or the sample electrode begins to operate. At that time, it takes generally several 100 milliseconds through several seconds from the start of operation of the matching circuit to its achievement of the full suppression of reflected power. Further, this necessary time varies in each of the repeated processes, and hence degrades the controllability and reproducibility. There is a difficulty in obtaining stably a desired doping density.
In particular, in the case of low density doping at a doping density of 1×1011 atm/cm2 through 1×1015 atm/cm2, the processing time is as short as several seconds through ten and several seconds. Accordingly, the processing is affected strongly by the variation in the reflected power.
Furthermore, when at least one of control parameters, such as gas species, gas flow rate, pressure and high frequency electric power is changed during the process of plasma doping with maintaining the generation of plasma, a large reflected power is liable to occur at the time of change. Variation in this reflected power is large, and thus the controllability and reproducibility in the doping density are degraded.